One common form of transducer utilized in sensing strains, such as representative of torque developed in a drive shaft, comprises a resistive source impedance strain gage. Such strain gages are well known and have their sensitivity expressed in volts output per volt of excitation so that strain measurements made therewith are extremely accurate and repeatable. Accurate laboratory calibrations of strain gages can be readily transferred to the field regardless of differences in the length and exact nature of the cabling used in the laboratory calibration and in the field installation. The effects of such cables are eliminated by temporarily replacing the transducer at the end of the cable with an electrical calibrator that produces a known voltage output per volt of excitation. Such elimination of the cable effects is readily effected where the calibrator has exactly the same output impedance as the transducer.
In using such a calibrator, the calibrator is adjusted to produce either a preselected percentage or a full scale output reading of the previously calibrated transducer. The system gain is then adjusted to full scale or an appropriate percentage of full scale and the transducer is then substituted for the calibrator.
A problem arises, however, when the source impedance of the transducer is not identical to that of the volt-per-volt calibration standard. Such failure of identity occurs where the transducer is not solely resistive. Such transducers having both resistive and reactive impedances occur, for example, where the resistive strain gage is connected in the circuitry by means of rotary transformers rather than resistive slip rings. While such rotary transformers are highly advantageous in many applications where the strain to be measured occurs on a rotating body, such transformers introduce a reactive component so that the transducer impedance is no longer purely resistive. Notwithstanding such undesirable inclusion of the reactive component, such rotary transformers have been used extensively because of the numerous advantages thereof, including elimination of the slip ring problems, including limited life, bridge balance drift due to shunting ring and brush wear, susceptibility to contaminuation, susceptibility to vibration, etc. Such rotary transformer devices, such as torquemeters, are extremely repeatable and, thus, if it were not for the above discussed disadvantages, would provide an ideal substitute for the slip ring systems of the past.
In addition to the inclusion of the reactive impedance in the rotary transformer torquemeters, it is common to utilize temperature compensation networks which cause a variation in the output impedance between different production devices. As a result of these factors, changes in the cable length or cable configuration cause the complex ratio of the transducer output to that of the associated passive calibration networks to vary at the load end of the cable. Thus, even though such systems are extremely repeatable for a given transducer/cable combination, each such combination must be calibrated accurately in order to provide desired high measurement accuracy in the use thereof. Thus if the transducer is replaced, or the cable is replaced or modified, calibration validity is compromised and recalibration thereof must be effected.